Known screens comprise two plates at least one of which is transparent. Between these plates are disposed liquid crystal molecules. To excite the liquid crystal molecules and to modify the polarity of a light ray which passes through them, the screen comprises transparent electrodes situated on internal faces of the two plates. One of these electrodes is commonly called the counter-electrode. A control signal, in general of low frequency, is applied between the electrodes to excite the liquid crystal molecules and thus obtain the desired display.
It is known to construct interactive communication terminals by disposing one or more transparent touch layers on the device, these being based on contact or on capacitive or inductive effect or the like. The device can then serve to delimit pressing zones of this pad according to a desired composition. The pressing of the layer is usually done by a finger of an operator. The drawbacks associated with these known methods relate to the cost of the sensitive component added to the screen and to the fact that the visual information provided by the device is inevitably altered by the add-on touch pads.
To remedy this problem, European patent EP 0 340 096 proposes that the surface of the counter-electrode of a liquid crystal screen be used as replacement for add-on touch layers. This counter-electrode has a significant area in relation to the area of the finger of the operator, this being well suited to the detection of a finger by capacitive effect. By way of example, such a screen allows the displaying of numerical characters each composed of seven segments which may either be lit or unlit. A counter-electrode of this type of screen generally covers one or more predefined locations of characters.
Patent EP 0 340 096 describes the application of a high frequency signal to the counter-electrode. The high frequency signal is attenuated by the presence of the finger of a user in proximity to the screen and opposite the counter-electrode. The detection of the attenuation of the high frequency signal makes it possible to detect the presence of the finger.
Patent EP 0 340 096 describes the independent application of the control signal and of the high frequency signal so as not to disturb one another; this independent application requires that one of the signals be interrupted in order to apply the other. This does not make it possible to modify the display and to detect the presence of the finger simultaneously.